Gram-positive microorganisms, such as members of the group Bacillus, have been used for large-scale industrial fermentation due, in part, to their ability to secrete their fermentation products into the culture media. In gram-positive bacteria, secreted proteins are exported across a cell membrane and a cell wall, and then are subsequently released into the external media usually obtaining their native conformation.
The folding of polypeptide chains depends upon the assistance of molecular chaperones and folding catalysts, such as disulfide bond isomerases, which accelerate specific steps in folding. (Hart) et al., 1995, Current Opinion in Structural Biology, 5:92–102). Disulfide bond isomerases can covalently modify proteins by catalyzing specific isomerization steps that may limit the folding rate of some proteins (PCT/US93/09426).
Disulfide bond isomerase catalyzes thiol/disulfide interchange reactions and promotes disulfide formation, isomerization or reduction, thereby facilitating the formation of the correct disulfide pairings, and may have a more general role in the prevention and premature misfolding of newly translocated chains. Disulfide bond isomerase interacts directly with newly synthesized secretory proteins and is required for the folding of nascent polypeptides in the endoplasmic reticulum (ER) of eukaryotic cells.
In spite of advances in understanding portions of the protein secretion machinery in procaryotic cells, the complete mechanism of protein secretion and the mechanisms associated with correct folding of secreted proteins, especially for gram-positive microorganisms has yet to be fully elucidated.